linux-kernel - Re: [PATCH v9 10/17] refcount: introduce __refcount_{add|inc}_not_zero

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Message-ID: <20250111121320.1656-1-hdanton@sina.com>
Date: Sat, 11 Jan 2025 20:13:18 +0800
From: Hillf Danton <hdanton@...a.com>
To: Suren Baghdasaryan <surenb@...gle.com>
Cc: akpm@...ux-foundation.org,
	peterz@...radead.org,
	willy@...radead.org,
	hannes@...xchg.org,
	linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	kernel-team@...roid.com
Subject: Re: [PATCH v9 10/17] refcount: introduce __refcount_{add|inc}_not_zero_limited

On Sat, 11 Jan 2025 01:59:41 -0800 Suren Baghdasaryan <surenb@...gle.com>
> On Fri, Jan 10, 2025 at 10:32 PM Hillf Danton <hdanton@...a.com> wrote:
> > On Fri, 10 Jan 2025 20:25:57 -0800 Suren Baghdasaryan <surenb@...gle.com>
> > > -bool __refcount_add_not_zero(int i, refcount_t *r, int *oldp)
> > > +bool __refcount_add_not_zero_limited(int i, refcount_t *r, int *oldp,
> > > +                                  int limit)
> > >  {
> > >       int old = refcount_read(r);
> > >
> > >       do {
> > >               if (!old)
> > >                       break;
> > > +
> > > +             if (statically_true(limit == INT_MAX))
> > > +                     continue;
> > > +
> > > +             if (i > limit - old) {
> > > +                     if (oldp)
> > > +                             *oldp = old;
> > > +                     return false;
> > > +             }
> > >       } while (!atomic_try_cmpxchg_relaxed(&r->refs, &old, old + i));
> >
> > The acquire version should be used, see atomic_long_try_cmpxchg_acquire()
> > in kernel/locking/rwsem.c.
> 
> This is how __refcount_add_not_zero() is already implemented and I'm
> only adding support for a limit. If you think it's implemented wrong
> then IMHO it should be fixed separately.
> 
Two different things - refcount has nothing to do with locking at the
first place, while what you are adding to the mm directory is something
that replaces rwsem, so from the locking POV you have to mark the
boundaries of the locking section.